l. Field of the Invention
This invention relates to a method for manufacturing ceramic substrates. More specifically, it relates to a method for producing ceramic substrates designed for mounting thereon semiconductor or integrated circuit chips.
2. Description of the Prior Art
FIG. 1 shows a flow chart of the conventional ceramic substrate manufacturing process. Manufacturing of a ceramic substrate starts with the preparation of a so-called green sheet. This green sheet is made by first forming a slip from a mixture of alumina powders with organic solvents, passing the slip through a blade with a predetermined gap, casting it into a sheet form and drying the formed sheet. This green sheet is then punched to form the guide holes which serve as a reference position in the succeeding steps. Through-holes are also punched out in the green sheet. These holes may be formed by drilling. On the surface of said green sheet is then formed a paste-like conductor by a usual means such as screen printing. The through-holes are filled up with the conductor in this operation. For forming a multi-layer ceramic substrate, a desired number of such green sheets are placed one upon another and press bonded under suitable heating and pressure, and the resulting laminate sheet is cut to a desired size and sintered at a temperature of one thousand and several hundred degrees centigrade.
According to the prior art, punching of the green sheet for forming the guide holes has been conducted after approximately one week standing of the green sheet after its preparation. This was necessary for aging the sheet till the rate of its dimensional change diminishes to an insignificant level since the green sheet is subject to dimensional change which begins soon after its formation and continues for a certain period. As seen from FIG. 2 showing the curves of dimensional change of a conventional green sheet, the amount of dimensional change of the green sheet diminishes with the lapse of time (days). It will be also seen that the amount of dimensional change differs greatly between X direction and Y direction in relation to the casting direction.
However, as noted from FIG. 2, the amount of dimensional change of the green sheet is fairly large even after approximately one week standing. This is prone to cause positional deviations in the ensuring steps, making it difficult to obtain a ceramic substrate of high precision. Several measures have been proposed and employed for overcoming this problem, such as extending the standing period of the green sheet; reducing the time used for the process from guide hole punching till lamination; and diminishing the size of the green sheet to reduce a number of substrates cut out from said green sheet.